Use 219 chain or #35 chain. The benefits/disadvantages of each have been discussed in various threads around- 219 is smaller pitch so you can use a smaller sprocket (not needed in your application)- 35 can be stronger. Both chains can be found in high quality, strong chain for racing go-karts. (both can also be found cheap and crappy...) There is even some o-ring 219 but it's quite expensive. Another advantage to #35 is that sprockets in all sizes and configurations are available from surplus center, mcmaster, etc., instead of 219 which is really only available in kart-oriented sizes. For greater torque handling you could go to 420 but it's not likely to be necessary for your application and it's very big and wide.
Are you planning to make a freewheeling drive (with the regen on the sevcon I'd expect not-) Right hand drive? Left hand drive? If left (on the disk brake hub) will you put a mechanical rear brake on it? Rim brake? Something else?
Check out the sprockets and adapters from these guys http://kingssalesandservice.com/top-hat-sprocket-adaptor/
Maybe recumpence Matt may have a "top hat" of his own, not sure what his current designs look like.
Yay chain drives. You'll get it all worked out. I like the plan of using the hub as a chain drive motor on the swingarm close to the pivot. I will look forward to seeing how it comes together!
Nice score on the ultra motor. The funny integrated spoke flange/side cover/ split down the middle of the hub gives it away! I'd expect 25 horsepower is going to be pretty demanding for that motor- got any cooling planned?